The Obama administration will likely soon release at least part of a 28-page secret chapter from a congressional inquiry into 9/11 that may shed light on possible Saudi connections to the attackers.

The documents, kept in a secure room in the basement of the Capitol, contain information from the joint congressional inquiry into "specific sources of foreign support for some of the September 11 hijackers while they were in the United States".

Bob Graham, who was co-chairman of that bipartisan panel, and others say the documents point suspicion at the Saudis.

The former Democratic senator from Florida says an administration official told him that intelligence officials will decide in the next several weeks whether to release at least parts of the documents.

The disclosure would come at a time of strained US relations with Saudi Arabia, a long-time American ally.

"I hope that decision is to honour the American people and make it available," Mr Graham told NBCs' Meet the Press on Sunday.

"The most important unanswered question of 9/11 is, did these 19 people conduct this very sophisticated plot alone, or were they supported?"

Tim Roemer, who was a member of both the joint congressional inquiry as well as the 9/11 Commission and has read the secret chapter three times, described the 28 pages as a "preliminary police report".

"There were clues. There were allegations. There were witness reports. There was evidence about the hijackers, about people they met with - all kinds of different things that the 9/11 Commission was then tasked with reviewing and investigating," the former Democratic congressman from Indiana said on Friday.

Fifteen of the 19 hijackers were citizens of Saudi Arabia. The Saudi government says it has been "wrongfully and morbidly accused of complicity" in the attacks, is fighting extremists and working to clamp down on their funding channels. Still, the Saudis have long said that they would welcome declassification of the 28 pages because it would "allow us to respond to any allegations in a clear and credible manner."

The pages were withheld from the 838-page report on the orders of then president George W. Bush, who said the release could divulge intelligence sources and methods. Still, protecting US-Saudi diplomatic relations also was believed to have been a factor.

Ben Rhodes, President Barack Obama's deputy national security adviser, said Mr Obama asked National Intelligence director James Clapper to review the papers for possible declassification.

"When that's done we'd expect that there will be some degree of declassification that provides more information," Mr Rhodes told reporters in Riyadh last week where Mr Obama met with King Salman and other Saudi leaders. The White House says the 28 pages did not come up during discussions.

Neither the congressional inquiry nor the subsequent 9/11 Commission found any evidence that the Saudi government or senior Saudi officials knowingly supported those who orchestrated the attacks that killed nearly 3,000 people. But Mr Graham, the relatives of victims and some politicians think there is reason to further probe possible Saudi links.

Mr Roemer said many questions remain about the roles of Fahad al Thumairy, an official at the Saudi consulate in Los Angeles who allegedly helped two of the hijackers find housing and transport after they arrived in Southern California.

Al Thumairy was later denied entry into the United States in May 2003 after the State Department alleged that he might be involved in terrorist activity.

Mr Roemer also wants to know more about Omar al Bayoumi, who was strongly suspected of being a Saudi spy and was alleged to have been helpful to the hijackers.

"We did not discover ... Saudi government involvement at the highest level of the 9/11 attacks," Mr Roemer said.

But he added: "We certainly did not exonerate the Saudis. ... Saudi was a fertile ground for fundraising for al Qaida. Some of these issues continue to be problems today. That's why we need to continue to get to the bottom of this."

The online 28pages.org, an Internet site pushing to get the documents released, points to another document declassified in July 2015 that outlined ways in which the commission could examine possible Saudi links.

The 47-page document lists several pages of individuals of interest and suggests questions that could be pursued. One name is suspected al Qaida operative Ghassan al Sharbi.

Al Sharbi, who was taking flight lessons in the Phoenix area before 9/11, was captured in 2002 in the same place in Pakistan as Abu Zubaydah, a top al Qaida trainer who was apprehended and waterboarded dozens of times by US interrogators.

The document said that after al Sharbi was captured, the FBI discovered some documents buried nearby. One was al Sharbi's pilot certificate inside an envelope from the Saudi Embassy in Washington, although it's unclear whether the license had been mailed by the embassy or if the envelope was simply being reused.

A CIA's inspector general report in June 2015 said there had been no reliable reporting confirming Saudi government "involvement with and financial support for terrorist prior to 9/11".

But it also that people in the CIA's Near East Division and Counterterrorism Centre "speculated that dissident sympathisers within the government may have aided al Qaida". The rest of the chapter, titled Issues Related To Saudi Arabia, is blacked out.

A bill directing the president to release the 28-page chapter was introduced in the Senate, and nearly three dozen Republicans and Democrats in the House are backing a similar resolution.